This invention relates to hydraulic actuation systems and in particular hydraulic actuation systems for automated transmission systems. In this context automatic transmission systems include mechanisms and/or methods for the controlled, automated actuation of the clutch and/or gears in the drive train of motor vehicles.
In automated transmission systems of, for example, the type disclosed in WO97/05410 or WO97/40300, whose content is expressly incorporated in the disclosure content of the present application, fluid pressure actuators are used to control actuation of a clutch actuator mechanism and/or a gear engaging mechanism. In accordance with WO97/05410, separate control valves are used to control the clutch actuator mechanism and the gear engaging mechanism.
WO97/40300 discloses a hydraulic actuation system in which a main control valve controls both the clutch actuation mechanism and, together with secondary valves, shift and select actuators of a gear engaging mechanism. The use of a single main control valve in this manner reduces the number of components, providing savings in the overall size and cost of the system. The design of the master control valve is however significantly more complicated, which reduces the cost savings.
Hitherto, hydraulic pressure for clutch actuation and for shifting gears has been supplied from a gas accumulator which is charged by means of an electrically driven pump. The accumulator provides an immediate supply of pressurised fluid, to disengage the clutch, upon initiation of a gear change. The use of an accumulator also enables the use of a smaller pump.
Typically the pressure required to operate the clutch will be of the order of 30 bar while the pressure required for gear shifting may be as high as 60 bar, but will generally be about 20 bar. However, in order to store sufficient volume for clutch actuation and shifting of gears, pressure in the accumulator must be as high as 60 bar. The pump has consequently to work at 60 bars to charge the accumulator in order to store the required volume of fluid.
Moreover, it is desirable during the shifting of gears to vary the force applied to the shift actuator, when, for example, engaging the syncromesh. This has been achieved hitherto by using a pressure transducer to measure the pressure of fluid supplied to the shift actuator and modulating the flow of fluid into the system using separate proportional flow control valves to maintain the correct pressure.
In accordance with one aspect of the present invention, a hydraulic actuation system for an automated transmission system comprises;
a hydraulic clutch actuator for controlling engagement of a clutch;
a gear engagement actuator for controlling engagement of a gear;
an accumulator;
an electric motor driven pump, an output of the motor driven pump being connected to the accumulator for charging thereof via a non-return valve, the pressure of fluid supplied by the pump being controlled by the current supplied to the electric motor; and
a main control valve for selectively connecting the accumulator to the hydraulic clutch actuator and/or the gear engagement actuator.
In this manner hydraulic fluid under pressure will immediately be available from the accumulator for disengagement of the clutch. Pressure of fluid to the gear engagement actuator may then be controlled by the pump. As a consequence the volume and/or pressure of the accumulator may be reduced. Furthermore the pressure supplied to the gear engagement actuator may be modulated by the pump, avoiding the need for flow control valves and the control system associated therewith.
According to a preferred embodiment of the invention a spring accumulator is used. The spring accumulator will also provide consistent compliance in the system enabling accurate pressure control.
A pressure transducer may also be included in the system and used in a closed feedback loop to control the pump to deliver the required pressure.